* {
    padding: 0;
    margin: 0;
}
a {
    text-decoration: none;
    color: black;
}
body {
    background-color: #f1f1f1;
}
header {
    overflow: hidden;
    width: 100%;
    line-height: 70px;
    text-align: center;
    height: 70px;
    background-color: #ccc;
    color: red;
    font-size: 50px;
    font-weight: 700;
    cursor: pointer;
}
section {
    width: 100%;
    height: 400px;
    /* background-color: pink; */
    position: relative;
    
}
section .top {
    width: 100%;
    height: 400px;
    margin: 0 auto;
    text-align: center;
    line-height: 400px;
    font-size: 100px;
    
}
section .mask {
    display: none;
    width: 100%;
    height: 400px;
    background: rgba(0, 0, 0, .4);
    position: absolute;
    top: 0;
    left: 0;
    font-size: 100px;
    text-align: center;
    line-height: 400px;  
    
} 
section:hover .mask {
    display: block;  
}
section:hover .top {
    display: none; 
}
.mask:hover a {
   color: skyblue;
}